Thermoelectric conversion technology is a technology of performing direct conversion between heat energy and electric energy by semiconductor materials, its principle is achieving thermoelectric electricity generation and thermoelectric refrigeration according to the Seebeck effect and the Peltier effect of the material. This technology has many advantages in application such as no pollution, no mechanical transmission, no noise, and high reliability, thus being widely used in recovery and utilization of industrial waste heat, special space power supply, miniature refrigeration devices, etc. In recent years, due to growing energy shortage and environmental pollution problems, the research of thermoelectric materials is getting more and more attention.
The optimum energy efficiency and high-low working temperature of thermoelectric materials are related to the essential properties of the materials, among which, the thermoelectric properties of the materials are determined by the dimensionless ZT values with a specific definition of ZT=S2σT/k, wherein S indicates the Seebeck coefficient, σ is the conductivity, T is the absolute temperature, and k is the thermal conductivity of materials. The higher the ZT value of the material, the higher the conversion efficiency of the thermoelectric energy will be.
Application of thermoelectric materials in the refrigeration field is mainly for micro-device refrigeration. On the basis of the Peltier effect, in the π-type device formed by connecting P-type and N-type thermoelectric materials via a current-conducting plate, when an electric current passes through it, the device absorbs heat and performs refrigeration at one side, and radiates heat at the other side. The performance parameters of the thermoelectric refrigerator mainly include refrigerating efficiency, maximum refrigerating capacity, and maximum temperature difference. Refrigerating efficiency is the ratio of the refrigerating capacity to the input power. In the working state where the optimal input current is applied, the maximum refrigerating efficiency is:
            η      max        =                  T        c                              T          h                -                  T          c                      ,                              (                      1            +                          Z              ⁢                                                          ⁢                              T                _                                              )                          1          /          2                    -                        T          h                /                  T          c                                              (                      1            +                          Z              ⁢                                                          ⁢                              T                _                                              )                          1          /          2                    +      1        ,wherein Tc is the cold junction temperature and Th is the hot junction temperature,
            T      _        =                            T          h                +                  T          c                    2        ,i.e., T is the average temperature of Tc and Th, and ZT is the average thermoeletric figure of merit of the thermoelectric refrigerating device.
The maximum refrigerating capacity refers to the refrigerating capacity when the device is in the best working condition and the temperature difference between the two junctions of the device is zero:
            Q              c        ·        max              =                  1        2            ·                                                  (                                                S                  p                                -                                  S                  n                                            )                        2                    ⁢                      T            c            2                          R              ,wherein SP is the Seebeck coefficient of P-type materials, Sn is the Seebeck coefficient of N-type materials, and R is the resistance of the refrigerating device.
When the device is operated in a state without adscititious heat load, the temperature difference between the cold and hot junction is:
            Δ      ⁢                          ⁢      T        =                                        (                                          S                p                            -                              S                n                                      )                    ⁢                      T            c                    ⁢          I                -                              1            2                    ⁢                      1            2                    ⁢          R                    k        ,wherein I is the input current, and k is the total heat conductivity coefficient of two junctions of the device.
If the thermoelectric refrigerating device is working in the corresponding optimum current, the temperature difference generated between the cold and hot junctions of the device is the maximum temperature difference:ΔTmax=Th−Tc=½ZTc2.The maximum temperature difference is only related to the ZT of the device.
One of the most important goals of scientific and technical workers is to find and pursue new thermoelectric materials with a high ZT value. In the present study of thermoelectric materials, researchers have proposed and found a series of new materials, mainly including skutterudite and clathrate systems of cage compounds based on the conception of “phonon glass-electron crystal,” oxide systems with layered structures, lead telluride materials with rock-salt structures, diamond structure systems of a wide bandgap type, Cu2Se materials with liquid-like properties, and low-dimensional structure materials such as nanowires, ultra lattice, film, and nanostructured bulk materials, etc. At the same time, researchers have also found some new methods and means to improve the performance of thermoelectric materials, for example, the thermoeletric figure of merit can be greatly improved by the following methods: increasing the Seebeck coefficient by introducing a resonance energy level near the Fermi level, introducing a complex band-structure near the level of determining performance transmission, realizing two-dimensional plane electron waves in block materials, filling single or multiple elements in a cage structure compound, and reducing the phonon modes on the basis of liquid-like effects, etc. The ZT value of the bulk materials have been improved obviously by realization of these new materials and new methods, with the maximum value reaching more than 1.5, and the energy conversion efficiency more than 10%. However, because all of these new materials are a single structure system, the structure thereof will not vary within the temperature range of application, which, to a certain extent, limits the development of a wider material system. In the applications of micro-device refrigeration, the material system with an excellent thermoeletric figure of merit near the room temperature is relatively simple, and at present, the materials of wider commercial application mainly include a bismuth telluride-based material, for example, as described by CN101273474A. This material, which is prepared at a high cost and by a difficult method, has a thermoeletric figure of merit near room temperature of about 1.0, and a refrigeration efficiency of about 5%, which limit the wide application of thermoelectric conversion technology. In addition, multicomponent thermoelectric alloys have been developed as new types of thermoelectric materials, such as a Cu2CdSnSe4 semiconductor nanocrystal as disclosed by CN101823702A.
CN102674270A discloses a method of preparing the Cu2Se thermoelectric material by a low-temperature solid-phase reaction. Compound Cu2Se has a simple chemical composition, and undergoes a reversible phase transition in the vicinity of 400 K. After the phase transition, the high temperature phase is a cubic anti-fluorite structure, and the copper ion moves though the gap between the octahedrons and the tetrahedrons of the host lattice, thus having a fast ion-conducting property. Therefore, the Cu2Se is a widely used fast-ion conductor. The room temperature phase has a complex structure i.e., a complex monocline structure of double or triple cycles along the [010] direction. In such double or triple cycles, the copper atoms are squeezed between the Se atoms of the host lattice, and the Se atoms are combined with each other by van der Waals forces, so that the room temperature phase material exhibits a layered structure. During the transition from the room temperature phase to the high temperature phase, a portion of the copper ions between the selenium atoms are transferred to the vacuum layer and correspondingly the structure is changed into a stable cubic structure. During this process, the transfer of copper ions brings structural fluctuations which may affect the changes of electronic structure, and the phase transition process brings additional scattering of carriers, thus significantly increasing the Seebeck coefficient of the material, reducing the thermal conductivity, and further improving the thermoelectric figure of merit ZT of materials. Therefore, the Cu2Se thermoelectric materials have a good prospect in industrial application. The introduction of the phase transition material system into the study of thermoelectric materials enlarges the material system of thermoelectric study, and also provides a possibility of realizing a higher performance material, thus having a great significance for the study of thermoelectric materials.